I've spent several years providing fact and expert witness testimony. Here are my random 1.5 cents on how to act in the courtroom;
this is in no way a complete guide on courtroom behavior - just my ramblings.

Bring some other reading material or something
to occupy your time while not in the courtroom. I’ve sat in many courthouses
for hours prior to actually taking the stand. Finished many books this way

Courtroom benches can be quite uncomfortable. Be aware of this.

Always arrive at least 30 minutes earlier than
normal, especially if the courthouse is an unknown to you.

Be calm.

Breathe.

Sit up straight in the witness chair. Do not
slouch.

Pronounce your words coherently and do not talk
quickly. If there is a microphone, use it.

Unless stipulated, be prepared to talk about
yourself – education, job history, certifications, job duties, papers published
– basically anything that will lay the foundation for you as an expert witness.

Be prepared to be challenged on your knowledge, education, and history.

When asked a question, take a breath, pause, and
then answer.

If the trial is a jury trial, look at the attorney
who is addressing the question to you, then when answering the question, look
at the members of the jury. Make eye contact with each and every one that you
can. Do this routinely.

Do not interrupt the attorney. Let the attorney
finish her sentence, pause, and then directly answer the question.

Be calm.

Things have the possibility to get a little
heated in the courtroom, but remember you are the expert in your field and are
called to testify for that very reason – you are the scientific expert.

It is ok to say “I don’t know” or “I don’t
recall”. Especially if you really don’t know or if you really cannot recall.

If there is an objection by an attorney, stop
what you are saying. Do not continue until the judge says it is ok to continue.

Never lie, misrepresent the science, or make up
any information.

Breathe

Humans do make mistakes. It may be hard to admit
this in court, but it’s a fact of life. I’ve gotten into a few good discussions
while on the stand about human errors.

The validated procedures we use in the field are
only as good as the people trained to do them.

There are checks and balances and redundancies
built into a reputable laboratory’s procedures for a reason. Don’t be afraid to
talk about them.

A good attorney will prep you for testimony. A
bad attorney will not.

Always maintain contact with the attorney that
has retained your services. If you have a question about the court process, do
not hesitate to ask.

If using figures or diagrams, please consult
with attorney beforehand.

Do not try to draw chemical structures or
anything else complex without a guide. Do not do it “off the top of your head”
while in front of the judge, jury, and attorneys.

Always remain calm on the stand. It is quite
easy to become frazzled when a defense attorney rapid-fires questions at you.
Again, let her finish the question, pause, take a breath, and answer
confidently and coherently.

Never leave the court until released by the
judge.

You know what you know. You are the expert. Do
not elaborate outside the area of your job expertise or scope. I am always
happy to say “That is outside the scope of my expertise”.